1. Field of the Invention
The present invention generally relates to image forming apparatuses such as laser printer and copying machine, and more particular, to an image forming apparatus improved in its toner supply operation.
2. Description of the Related Art
For copying machines, two types of developer are available; one is of unary system and the other is of binary system. The developer of binary system is composed of toner and carrier which have different polarities. The toner serving as colorant is absorbed by a photoreceptor, thus allowing an electrostatic latent image on the photoreceptor to emerge clearly. In the developer of binary system, therefore, carrier is not consumed at all but toner is always exhausted.
In a conventional copying machine, toner is fed from a toner container and supplied to a developing unit through supply means. Then, detection is made on presence or absence of toner in the toner container. When toner is used up, a "toner empty lamp" is lighted and an operator is informed that the toner container is emptied so as to require the operator to exchange the emptied toner container.
However, conventional copying machines can continue copying operation even when the toner empty lamp is lighted. When copying operation is continued with the toner empty lamp lighted, the toner concentration in the developing unit is gradually reduced, resulting in a reduced image density of copies. This problem is attributable to the fact that presence or absence of toner in the toner container is detected only indirectly by using a toner concentration sensor in a developer tank, and not detected directly in the toner container. That is, there occurs a time delay between the time toner in the toner container is actually used up and the time the toner concentration begins to decrease. Therefore, especially when the detection of emptiness of the toner container is delayed and copying operation is continued after the toner empty lamp is lighted, the problem becomes significant. Furthermore, if the toner container is exchanged after the image density has been considerably reduced and copying operation is resumed with toner being supplied as usual, it takes some time for the toner concentration to reach a predetermined value. As a result, the initial several copies taken during that period show a low image density and poor image quality.